1. Field of the Invention
The present invention relates to an all solid type electrochromic display element and its use.
2. Description of the Prior Art
In some particular substances such phenomenon can be observed that when a voltage is applied to the substance, an oxidation-reduction reaction takes place in it and the substance becomes reversibly colored or bleached. This phenomenon is called electrochromism in the art and substances which exhibit such phenomenon are called electrochromic materials.
For more than fifteen years, many attempts have been made to develop novel display elements using such electrochromic materials, and to use the novel display element for displaying numerals in clocks, watches or electronic calculators. Some attempts have resulted in developing such display elements which are able to repeat coloring and bleaching in response to the application to and removal from the element of voltages. For example, there is already known in the art such all solid type electrochromic display element comprising a transparent electrode film (cathode), a thin film layer of tungsten trioxide, an insulating film layer formed of, for example, silicon dioxide and a second electrode film (anode) arranged in that order on a glass substrate to form a multi-layer structure.
When a voltage is applied to the display element, the tungsten trioxide (WO.sub.3) layer becomes colored blue. The blue color disappears when there is applied a voltage of a polarity opposite to that of the first applied voltage. The layer of WO.sub.3 becomes again colorless. Although the mechanism of this coloring and bleaching phenomenon has not yet been fully ascertained, it has been known that a minor content of water in the WO.sub.3 layer and the insulating layer plays a dominant role in coloring and bleaching of the WO.sub.3 layer. It is likely that the coloring reaction proceeds as follows: ##STR1##
As seen from the above reaction formula, the known display element has the following drawbacks:
(1) The coloring reaction is accompanied with an unfavourable side reaction, that is, generation of oxygen gas, by which the water contained in the element is consumed.
(2) Since no water is produced during the bleaching reaction, the consumed water must be supplied from the atmosphere in order to repeat coloring. This brings forth another problem that the reproducibility of coloring in this type of display element is not stable but variable depending on the content of mositure in the atmosphere.
To overcome the above drawbacks there has recently been proposed a novel all solid type electrochromic display element which needs no supply of moisture from the atmosphere (cf. Japanese Patent Application Laid Open No. 73,749/1977). In this novel electrochromic display element, the same amount of water as consumed during the coloring reaction is produced by the following bleaching reaction. Therefore, it is possible to repeat coloring and bleaching without any external supply of water. Intensity of color developed in repeating the reaction remains always constant and is never affected by the moisture content in the atmosphere.
The known novel display element is basically composed of a transparent electrode layer, a thin film layer of electrolytic reduction electrochromic substance such as WO.sub.3, a thin film layer of electrolytic oxidation electrochromic substance such as Cr.sub.2 O.sub.3 and an opposite electrode layer laminated in that order.
In the above mentioned publication however, it is also disclosed that the color, once developed by an application of voltage on the display element, gradually disappears due to natural discharge after removing the voltage. According to the teachings made in the publication, this bleaching phenomenon may be excluded by providing a thin film layer of insultating substance such as silicon dioxide or magnesium fluoride. This insulating layer may be disposed in any layer position between the transparent electrode and the opposite electrode. The provision of such insulating layer gives the display element an ability to retain the color even after releasing the voltage (this ability is referred to as "memorability"). In other words, for this type of known display element it is essential to give it memorability by providing a particular insulating layer. We, the inventors of the present application, suppose that the insulating layer may allow proton (H.sup.+) and hydroxyl (OH.sup.-) ions to move freely through it although the layer is not a good conductor for electrons.